Basal cell carcinoma of the skin is by far the most common type of cancer in the United States, representing nearly one half of all newly diagnosed malignancies. Although these tumors infrequently cause cancer mortality, they are associated with significant morbidity due to local invasion and tissue destruction. Basal cell carcinomas are extremely variable in gross and microscopic appearance and biological behavior, and a common histopathologic classification system divides the tumors into five main categories which differ in aggressiveness, prognosis, and response to therapy. My laboratory isolated a gene for hereditary basal cell carcinomas (human patched; gene symbol PTCH) and showed that this gene is mutated in sporadic BCCs as well. Exactly analogous to the two-hit paradigm established for retinoblastoma, almost all sporadic basal cell tumors have two somatically-derived, inactivating PTCH mutations. PTCH is a member of the hedgehog signal transduction pathway. The few BCCs lacking mutations in PTCH have activating mutations in smoothened (SMO), another member of the hedgehog pathway whose protein is normally repressed by the patched protein. All subtypes of BCCs have mutations in PTCH or SMOH, indicating that mutations in one or the other of these genes are essential to the development of BCCs but do not dictate the histologic subtype. Furthermore, other genetic alterations known to occur in BCCs, such as p53 or RAS gene mutations, do not correlate with variation in biologic behavior. The purpose of this study is to identify the genetic causes and molecular correlates of different histologic subtypes of basal cell carcinomas. Specific aims are to 1) determine if genetic variation in members of the hedgehog pathway downstream from PTCH and SMO contribute to variation in biological behavior of these tumors, and 2) as an indirect method of examining other genetic and epigenetic phenomena in BCCs, use microarray analysis to try to classify these tumors and identify a set of genes whose expression predicts their biological behavior.